The present invention refers to a method and an apparatus for mass producing containers made of thermoplastic material, particularly polyethylene terephthalate (PET) and polypropylene (PP), for applications involving also their being filled with liquids at an elevated temperature and/or containing CO.sub.2 (carbon dioxide) gas.
In the field of technologies and machines for producing containers of the above cited kind there are a number of developments and improvements aimed at obtaining production methods and related apparatuses capable of producing the containers in an increasingly robust, reliable, cost-effective, versatile manner, and to increasingly high quality standards, in a highly competitive industrial context of very large-scale production.
Such production processes are known to be able to be schematically divided into two basic types, i.e. single-stage and two-stage processes, wherein the present invention applies only to single-stage methods and apparatuses.
Single-stage processes are so named due to the fact that they are capable of producing the so-called parison or bubble, transferring such parison or bubble from the injection or extrusion mold (after allowing it to cool down to any appropriate temperature) to a conditioning station where it levels evenly off at a temperature of preferred molecular orientation. The parison or bubble is then transferred to a blow-molding die, where it is molded into its ultimate, desired shape.
Inherent to any single-stage process is the fact that an uneven heat distribution is unfailingly brought about in the cross-sectional direction of the wall thickness of the parison when the latter is transferred from the injection or extrusion mold. A number of methods and processes have been patented concerning the cycle times and the temperatures of the parison when this is extracted from the injection mold, in an attempt optimize them.
The patent literature covering single-stage processes discloses in all cases a final forming or molding of the container of thermoplastic material which is in some way or other carried through a conditioning station to reach an even wall temperature throughout the cross-section of the same wall, the temperature corresponding to the preferred molecular orientation temperature of the thermoplastic resin.
In particular, the improved solution disclosed in the U.S. Pat. No. 4,372,910 and U.S. Pat. No. 4,470,796 granted to Stroup, the preferred embodiment of which has been implemented in a single-stage injection-blow molding machine made by Van Dorn Plastic Machinery as generally disclosed in the patent specification to Stroup.
Such patents substantially describe and claim a process and an apparatus for producing hollow plastic bodies, typically bottles, in which the parison is obtained by the continuous extrusion of a flow of thermoplastic resin, particularly PET, in a plurality of multiple molds. The result of the invention disclosed in the patents is the elimination of the limitation placed on productivity by the fact that the parison molding operation, including the time taken by the plastic in the mold to cool down, is considerably longer than the subsequent blow molding operation. Since these operational times occur in series, i.e. in sequence, this fact causes in practice the blow molding rate to be slowed down so as to match the slower rate of the parison molding operation.
The improvement disclosed in the above cited patent consists, among other things, in using a plurality of multiple molds which are grouped into at least two distinct groups of molds. The parisons, produced in all molds belongs to a definite group of molds, are blow molded, as grouped on a mold-by-mold basis, in a single respective blow molding machine.
In this manner, the duration of the entire blow molding phase is virtually slowed down through the multiplication of the related production, so that the final overall rate of the blow molding phase is synchronized with the parison molding phase, thereby obtaining an elimination of the idle, i.e. waiting times and the associated losses in productivity.
However, such a solution does not solve the problem associated with the long parison molding times which keep conditioning the overall cycle time due in particular to the time required to cool down the parisons in the respective injection molds.
In order to reduce such a molding time a solution is known from U.S. Pat. No. 4,382,905 to E. I. Valyi, and granted to American National Can Company, which consists in reducing the parison molding time through the introduction of an additional holding phase between the molding phase and the parison conditioning phase.
In the holding phase, the parison is removed from its injection mold and transferred as quickly as possible into a special holding mold before the regular cool-down time of the injection mold is elapsed, but after allowing the temperature to adequately decrease so as to minimize the risk of crystallization, indicated at approximately 200.degree. C., and approach the orientation temperature.
The parison is then kept in the additional holding mold until the temperature of the parison decreases to the orientation temperature, i.e. approximately 95.degree. C.
The parison is eventually removed from the mold and transferred into the conditioning station and the subsequent cycle according to well-known methods.
The purpose of such an invention is to increase the productivity of the production process by reducing the dwelling time of the parison in the injection mold and, therefore, increasing the rate and, as a result, the overall productivity.
The time that is saved in this way is then spent in the additional holding mold. However, being that the latter is arranged in series, the rate of all process phases can be advantageously accelerated, thereby obtaining an improvement in the overall productivity of the process.
However, the improvement that can actually be brought about in such a manner is rather modest owing to the limited maneuvering speed of the molds concerned, due to their inertia and various other factors. Furthermore, an apparatus modified in such a manner is more expensive and complicated in its construction, as well as more delicate and maintenance-intensive in, its practical use due to its greater complexity associated with the addition of a second set of molds.
In order to mitigate the problem represented by the slowness of the molding process of a thermoplastic resin due to the time required by the same resin to shrink when injected into the mold and during cooling down, the U.S. Pat. No. 3,709,644 granted to Farrell Patent Company discloses a method based on the arrangement of a cylindrical element, with an inner piston sliding in the cylinder, arranged in parallel with respect to the mold and adapted to exert an additional pressure on the fluid resin contained in the mold during the cool-down phase, as well as a valve adapted to separate the plastified-resin injector or extruder element from the mold and compactor during the cool-down phase of the resin in the mold.
Although such a solution is actually effective in enhancing the quality of the end product and reducing the cycle time, albeit in a limited way, the fact remains that it is anyway necessary to spend the entire time needed to fill the mold and, subsequently, to compress the resin in the same mold, as well as to allow for the parison to duly cool down, such aggregate time being unavoidably arranged to occur in series and in the same injection mold, with the result of a practically insignificant improvement in overall productivity.
Such a situation could actually be improved if the time taken to complete the slowest phase, i.e. filling the resin in the parison injection mold and allowing it subsequently to cool down, could be reduced without altering either, the general process or the characteristics of the end product to any considerable extent. As far as the cool-down time is concerned, this is practically constrained by the fact that a sufficiently low temperature shall be reached on the parison body, since the subsequent conditioning phases, which are carried out with generally known techniques, and in particular by means of IR heating techniques, to the main purpose of heating up the collar of the parison and even it out at an optimum temperature (approximately 115.degree. C.), have the effect of bringing about also a rise, beyond generally acceptable values, in the temperature of the body of the parison, so that the need arises to begin the conditioning phase in the presence of a sufficiently low temperature. This therefore involves a definite cooling of the parison when the latter is still in the mold and, as a result, a minimum cooling time that cannot practically be reduced, thereby placing a limitation on the possibility of reducing the cycle time.
Owing to all these considerations, it is therefore a main purpose of the present invention to use a single-stage method and apparatus to produce a container of thermoplastic resin that is thermally stable, capable of being filled with both hot and/or carbonated or non-carbonated beverages. The method and apparatus must be capable of enabling the total occupation time of the injection mold by the parison to be reduced through a decrease in the time required to cool it down, reliable, cost-effective and easily implementable with readily available techniques.
These and further objects of the present invention will become apparent to those who are skilled in the art from reading and understanding the description that is given below.